Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. Such formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer, and thus, the porous layer forms a reservoir in which hydrocarbons are able to collect. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In conventional drilling processes, a drill bit is attached to a series of pipe sections referred to as the drill string. The drill string is rotated and, as the drilling progresses, it is extended by adding more pipe sections. Larger diameter pipes, or casings, also are placed and cemented in the well to prevent the sides of the well from caving in. Once an appropriate depth has been reached, the casing is perforated at the level of the oil bearing formation. If necessary, various completion processes then are performed to enhance the ultimate flow of oil from the formation. The drill string is withdrawn and replaced with a production string. Valves and other production equipment are connected to the well so that the hydrocarbons may flow in a controlled manner from the formation, into the cased well bore, and through the production string up to the surface for storage or transport.
As a well bore is drilled deeper and passes through hydrocarbon producing formations, the production of hydrocarbons must be controlled until the well is completed and the necessary production equipment has been installed. The most common way of controlling production during the drilling process is to circulate a drilling fluid or “mud” through the well bore. Typically, the fluid is pumped down the drill string, through the bit, and into the well bore. The hydrostatic pressure of the drilling fluid in the well bore relative to the hydrostatic pressure of hydrocarbons in the formation is adjusted by varying the density of the drilling fluid, thereby controlling the flow of hydrocarbons from the formation.
Drilling fluids, however, are used for a variety of other purposes. As the drill string is rotated and the bit cuts through the earth, a tremendous amount of heat and large quantities of cuttings are generated. The drilling fluid serves to lubricate and cool the drill bit. As it is recirculated back up the well bore, the drilling fluid also carries cuttings away from the bit and out of the well bore. The drilling fluid also helps stabilize uncased portions of the well bore and prevents it from caving in.
Traditionally, drilling has been conducted in an overbalanced condition, that is, the hydrostatic pressure of drilling fluid in the well bore exceeds the pressure of hydrocarbons in the formation. Hydrocarbons, therefore, are prevented from flowing into the well bore. This avoids the risk that the well will blow-out and damage the environment and drilling equipment or injure those working on the drilling rig.
A major consequence of overbalanced drilling operations is that drilling fluid can flow from the well bore into the formation. That flow of fluid at relatively low levels is referred to as seepage and, at higher levels, as lost circulation. Seepage, and especially lost circulation, in turn may have several deleterious and costly effects. First, and obviously, any drilling fluid that flows into the formation must be replaced in order to maintain circulation of fluid through the well. The amount and cost of drilling fluid required to drill the well, therefore, is increased.
Moreover, drilling fluids typically comprise a variety of additives designed to improve the chemical and physical properties of the fluid. Seepage and lost circulation of drilling fluid necessarily carries with it whatever components are in the drilling fluid. It also carries fine cuttings generated by the drill bit. The cuttings, and many of the other components in the drilling fluid, however, can decrease the permeability of the formation. It then is more difficult for oil to flow from the formation once drilling is completed and production is started. Decreased permeability also may require acidizing or fracturing the hydrocarbon bearing formation to enhance production from the formation, which will further increases costs.
At high levels of lost circulation, differential sticking also may occur. That is, the drill string will be pulled against the wall of the bore hole by fluid flowing into the formation. Once stuck, the drill string can no longer rotate, and it is often difficult and time consuming to free the string so that drilling may resume.
The problems associated with seepage and lost circulation may be addressed by adjusting the density of the drilling fluid. Drilling fluids most commonly are high-density dispersions of fine, inorganic solids, such as clay and barite, in an aqueous liquid or hydrocarbon liquid. The density of the fluids may be controlled by the amount of solids added and, therefore, adjusted to balance the hydrostatic pressures at the interface between the well bore and the formation. Seepage and lost circulation and their attendant problems also may be addressed by the formation of a filter cake on the wall of the well bore or by the addition of filtration control and seepage control additives designed to physically impede the flow of fluid into the well bore.
Such drilling fluids are suitable for use in a wide variety of hydrocarbon bearing formations. In many formations, however, the hydrostatic pressure of hydrocarbons in the formation is relatively low, often because the formation is depleted. Many drilling fluids are simply too heavy for low pressure formations. They can significantly overbalance the well, allowing excessive amounts of drilling fluid to flow into the formation. The problems caused by seepage and lost circulation are exacerbated when a low pressure formation is also relatively fragile, such as are the fractured limestone formations found in the breccia of the Paleocene in many parts of the world. Fragile formations may be excessively fractured by the hydrostatic pressure of drilling fluid flowing into the formation and carry even more materials into the formation that will diminish its permeability. Seepage and lost circulation materials, in particular, if they are carried into the formation can cause extensive damage to the formation.
Accordingly, it is often preferable to drill through formations that are highly permeable, that have low pressures, or that are fragile in a near balanced or underbalanced state. That is, the hydrostatic pressure of the fluid in the well bore will be approximately equal to or less than the hydrostatic pressure of the formation, and various lower density drilling fluids have been developed for such purposes.
For example, low density diesel-water emulsions have been used as drilling fluids in fractured limestone formations. Those fluids comprise an emulsion of from about 75 to 85% diesel and from about 25 to 15% water and may have densities as low as about 7.0 lb/gal. Though lighter than dispersed solids formulations, those diesel-water emulsion fluids still are too heavy for such formations. Lost circulation can range from 3,000 to as high as 100,000 barrels of drilling fluid per well. Especially in those quantities, lost circulation greatly increases the costs of drilling fluid, complicates the logistics of supplying drilling fluid to the rig, and can cause extensive damage to the formation.
The effective density of drilling fluids may be lowered somewhat by aerating the fluid. Such fluids typically consist of conventional clay or polymer fluids lightened by injecting air, nitrogen or carbon dioxide. They may have densities as low as about 6.2 lb/gal, which may be lowered further, to around 5.8 lb/gal, by the addition of plastic or glass micro-spheres. Aerated fluids, however, still are too heavy for use in extremely low pressure, fragile formations without substantial losses. For example, in fractured limestone formations such as those in the Cantarell field offshore of Mexico, the drilling fluid must have a density of from about 4.2 to about 5.0 lb/gal in order to balance the well.
Such densities may be achieved by using foamed drilling fluids. They typically comprise a surfactant solution with gas dispersed therein. The surfactant acts to stabilize the gas dispersion. For environmental reasons, aqueous systems are preferred, and they typically include a polymer to improve the rheological and thioxotropic properties of the foam. Many types of foamed fluids are known, such as the aqueous, polymer based foamed drilling fluids disclosed in U.S. Pat. No. 5,706,895 to R. Sydansk. Foamed drilling fluids chemically are more complex and, therefore, their chemical and physical properties are somewhat more difficult to control.
In general, however, such foamed drilling fluids perform quite well in drilling operations and offer several advantages over traditional suspended solids drilling fluids. For example, the density of the foam may be controlled relatively easily by adjusting the gas injected into the foam. Also, the ability of foamed drilling fluids to carry cuttings away from a drilling bit is much greater than that of liquid drilling fluids. More effective removal of cuttings allows drilling to proceed at a faster pace, thereby reducing the time and expense of drilling. Moreover, when used at near balanced or underbalanced conditions, foamed drilling fluids can effectively prevent damage to even highly fragile, highly permeable formations.
Foamed drilling fluids are prepared by mixing a liquid phase, such as a polymer-surfactant solution, and a gas phase, such as nitrogen. Typically, this has been done by high velocity mixing of the phases or by injecting gas into the liquid phase through a small orifice. Most commonly, the foam is generated at the surface and then pumped into the well bore. It also has been suggested that drilling fluids may be foamed by pumping separate liquid and gas streams through a drill string to a downhole foam generator.
Foamed drilling fluids, therefore, typically require a source of gas such as nitrogen and various additional equipment that is not needed in conventional liquid circulation systems. For example, if liquid nitrogen is used, special tanks and equipment for cryogenically storing and handling the liquid nitrogen are required. Alternately, nitrogen membrane units may be used to produce nitrogen gas, although the gas produced thereby is only approximately 95% pure. Foam circulation systems also may include compressors, storage tanks, air pumps, foam generators, and other equipment beyond that commonly employed for circulating liquids. Moreover, unlike many other drilling fluids, which are hydraulic, foamed fluids are pneumatic. Special pneumatic pumps and control heads may have to be used to pump or otherwise control the foam in the well bore. Thus, systems for preparing and circulating foamed drilling fluids are relatively costly and require more maintenance, control, and logistical support than those required for more traditional suspended solids drilling fluids.
Such problems are exacerbated in offshore drilling operations where maintenance and logistical support is more difficult and costly. Space also is at a premium in offshore operations. On land, there usually is adequate space for additional equipment. Offshore, however, valuable space on the drilling rig deck is required, or it may be necessary to provide a barge or support boat to accommodate a foam circulation system. That can add considerable cost to the drilling operation.
An object of this invention, therefore, is to provide drilling fluids and, in particular, low density foamed drilling fluids that may be used in formations that are highly permeable, that have low pressures, or that are fragile without substantial lost circulation.
It also is an object to provide low density foamed drilling fluids having chemical and physical properties suitable for use in drilling operations.
Another object of this invention is to provide low density foamed drilling fluids that may be more easily and economically prepared and circulated in drilling operations.
Yet another object is to provide systems for circulating low density foamed drilling fluids that are simpler and require less specialized equipment and space for their installation and operation.
It is a further object of this invention to provide such drilling fluids and systems wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.